1. Field of the Invention
The present invention relates to a machine tool which comprises at least one working spindle with a tool-holding fixture for tool holders carrying tools, and a tool changer with a gripper for the tool holders in order to transfer the tool holders between their magazine position, which is spaced apart from the tool-holding fixture, and their working position in the tool-holding fixture, there being arranged in the working spindle a clamping system with a clamping apparatus, by means of which clamping apparatus a tool holder inserted into the tool-holding fixture is fixed.
The invention furthermore relates to a method for changing tools on a machine tool that comprises at least one working spindle with a tool-holding fixture for tool holders carrying the tools, and a tool changer with a gripper for the tool holders in order to transfer the tool holders between their magazine position, which is spaced apart from the tool-holding fixture, and their operating position in the tool-holding fixture, a clamping system being arranged in the working spindle, which clamping system fixes a tool holder, which is inserted into the tool-holding fixture, by means of a clamping operation and releases it by means of a release operation, wherein                a) a tool holder which is in its magazine position is inserted into the tool-holding fixture by movement of the gripper,        b) the clamping system grasps the tool holder at its upper end inserted into the tool-holding fixture and fixes it in the tool-holding fixture by means of the clamping operation, the tool holder being released from the gripper and the working spindle being brought up to its operating speed, and        c) in order to remove the tool the working spindle is braked, the clamping system carries out the release operation, and the tool holder is ejected into the gripper which then transfers the tool holder into its magazine position.        
2. Related Prior Art
Numerous machine tools of this type and methods for changing tools are known from the prior art.
DE 41 17 701 A1 describes a machine tool, in which a multiplicity of tool changers are distributed around the working spindle and each bear a gripper into which a tool holder carrying a tool is inserted. In this manner, there are as many tools in their magazine position, which is spaced apart from the tool-holding fixture in the working spindle, as there are tool changers.
In order to insert a tool into the tool-holding fixture, the tool changer is extended, in which case, on account of the parallelogram guidance, the gripper with the tool holder comes to rest under the working spindle. The tool changer is then moved upwards in the axial direction via a sleeve carrying all of the tool changers, as a result of which the tool holder is inserted into the tool-holding fixture. A clamping system is provided in the working spindle and now grasps the tool holder and fixes it in the tool-holding fixture.
After the tool holder is connected in a rotationally fixed manner to the working spindle, the latter is brought up to its operating speed. In this case, the gripper remains at the tool holder, with corresponding measures being provided so that the tool holder comes free from the gripper during operation.
If a new tool is to be inserted, the working spindle is braked until it reaches a coupling speed at which the clamping system is released and the tool holder is ejected from the tool-holding fixture and is again picked up by the gripper. The gripper then pivots the tool holder back into its magazine position while, in a push-pull operation, another tool holder is inserted by the corresponding tool changer into the tool-holding fixture and is fixed there with the aid of the clamping system.
DE 102 25 143 A1 discloses a machine tool which has two tool changers which, in a push-pull operation, interchange tools between a machine-tool magazine and the tool-holding fixture in the working spindle. The machine-tool magazine is arranged in a positionally fixed manner while the two tool changers are arranged on the spindle head carrying the working spindle. The spindle head moves into a changing position where the one tool changer removes the previously used tool from the tool-holding fixture while the other tool changer has already removed a tool which is to be inserted from the tool magazine where the tools are stored in their magazine position.
In the case of this machine tool, the gripper does not remain at the tool during use. If a tool is to be removed, the working spindle is first of all braked until it comes to a standstill before the gripper can grasp the tool which is thereupon ejected so that it can be placed in the tool magazine.
In the case of the method described last and the machine tool described last it is disadvantageous that the working spindle always first has to be braked to a standstill before the release operation for the clamped-in tool can begin. This is because there is the risk, when the clamping system is released, of the tool holder falling out of the tool-holding fixture, for which reason the clamping system is only released when the gripper has again grasped the tool. However, these method steps are time-intensive, which prolongs the time which is required for removing a tool.
Also during the clamping-in operation, with the moving away of the gripper a period of time has to be waited here until the clamping system has securely clamped in the tool holder, which likewise costs time.
Overall, the tool-changing time in the case of the known machine tool and in the case of the known method is found to be too long, since the tool-changing time enters considerably into the chip-to-chip time which is a measure of the machining speed of the machine tool. This is because the longer a tool change takes, the longer it also takes until the inserted tool is again in engagement with the workpiece. However, in particular in mass production, it is desirable to work with as short chip-to-chip times as possible.
One possibility of shortening the tool-changing time is to leave the gripper on the tool holder also during machining, i.e. to use a tool changer which moves together with the working spindle, as is the case in DE 41 17 701 A1 mentioned at the outset. However, this is not only structurally complex, this also increases the moving masses because a large number of tool changers have to be moved at the same time, with the storage capacity for tools also being restricted here.
However, even here a noticeable period of time is required until the tool holder can be ejected into the gripper and removed. Although the tool-changing time is reduced once again as a result of the fact that the tool holder is ejected into the gripper when the working spindle is still rotating at coupling speed, it nevertheless continues to be endeavored even here to further shorten the tool-changing time.